I also wanted to try out my Hoya CIR-PL HD filter. What a polarizing filter is supposed to do is remove reflections from non-metallic objects where the sun is coming from the side. It's also used to deepen blues of the sky. Here's an example without the circular polarizing filter.

And here with the circular polarizing filter.

That's a pretty dramatic difference. But here's an example where it didn't seem to help that much. Here's without the polarizing filter.